The present invention relates to a vibration diagnosis method and apparatus for rotary machines and, more particularly, to a vibration diagnosis method and apparatus suitably used for the vibration diagnosis of turbines and generators in thermal power plants and nuclear power plants, or the like rotary machines.
Recently, the capacities of steam turbines and generators in power plants are becoming greater and, accordingly, the size of the rotor of the turbine or generator is becoming larger. Due to the limitation in the shaft stress, the increase of the capacity of such rotary machines cannot be achieved by an increase of the shaft diameter solely. Namely, it is necessary to increase the length of the rotor shaft. The increased shaft length, however, imposes various problems such as reduction of shaft rigidity which in turn tends to produce vibration undesirably.
On the other hand, it is recognized that any failure in turbine or generator having large capacity causes serious social and economical problems, and requires much money and time for the repair. For these reasons, there is an increasing demand for the prevention of failure in these rotary machines.
It is also a current tendency to use a power plant of comparatively large capacity for adjustment of load to absorb the peak load. For such purpose, the power plant of comparatively large capacity has to be started and stopped frequently. This in turn gives a rise to a demand for enhanced safety in the frequent starting and stopping of the power plant. Vibration monitoring in such rotary machines has gained in importance as an effective measure for safe operation of these rotary machines. More specifically, it is required not only to monitor the level of the vibration but also to investigate and diagnose the vibration phenomenon and the cause thereof.
In the operation of these rotary machines, the vibration due to unbalanced mass is generated in synchronism with the revolution speed. This vibration is important as an object of the diagnosis, because it is generated most frequently.
The vibration due to unbalanced mass can be diminished by obtaining a balance of mass. At the beginning period of use of the machine, therefore, it is possible to reduce unbalance of mass caused in the process for manufacturing and assembling the rotor.
Unfortunately, however, the level of the vibration becomes greater as the rotary machine is used long, because of generation of unbalanced mass due to bow of the rotor, contact (rubbing) of rotor with a stationary part, losing of part or member of the rotor or other reasons which may occur during the operation of the rotary machine.
Among these reasons, the losing of the rotor parts such as blade, bolts or the like causes an abrupt increase of the vibration amplitude. It is, therefore, very important to discriminate this phenomenon caused by an abrupt generation of mass unbalance.
A change in the load applied to the turbine rotor or change in the electric load on the rotor of generator may cause a thermal unbalance in the rotor to produce a bow of the rotor generally referred to as "thermal bow".
The generation of such thermal blow is equivalent to a generation of an unbalanced mass in the rotor, and also causes a vibration.
Thus, although the vibration due to accidental generation of unbalanced mass and vibration due to thermal bow resemble each other, different countermeasures have to be taken depending on the causes of the vibration, i.e. whether the vibration is attributable to an abrupt generation of unbalanced mass or to a generation of thermal bow.
Therefore, in order to prevent any accident in the rotary machine, as well as to rationalize the maintenance, it is quite a rational measure to promptly and correctly discriminate the cause of the abnormal vibration of rotor when such a vibration has taken place.
Hitherto, such a method has been adopted as to discriminate the cause of abnormal vibration making use of the difference in frequency. However, since the vibration due to abrupt generation of mass unbalance and the vibration due to thermal bow have equal frequency, it is not possible to effectively discriminate with this method the cause of the vibration taking place in the rotary machine.